A high-temperature gas reactor has a reactor core, into which fuels for the high-temperature gas reactor are introduced, which reactor core is made of graphite that has a large thermal capacity and keeps its crystalline structure in good condition at high temperatures. The high temperature gas reactor employs, as coolant gas, a gas such as helium gas, which is assessed as being safe because helium gas does not react even at high temperatures. The employment of helium gas makes it possible to take the coolant gas away safely even when the temperature around the outlet of the coolant gas is high. Therefore, the coolant gas, which has been heated up to a high temperature of about 1000° C., is used as a safe heat source in a wide variety of fields such as hydrogen production and chemical plants, as well as power plants.
Fuels for the high-temperature gas reactor typically comprises a fuel kernel and a coating layer with which the fuel kernel is covered. The fuel kernel is a small particle with a diameter of about 350 to 650 μm, made by sintering uranium dioxide into a physical state like ceramics.
The coating layer generally comprises concentrically laminated sub-coating layers. When the coating layer has four sub-coating layers, they are called “the first sub-coating layer”, “the second sub-coating layer”, “the third sub-coating layer”, and “the fourth sub-coating layer” from the sub-coating layer adjacent to the fuel kernel. The diameter of the particle comprising the fuel kernel and four sub-coating layers is typically about 500 to 1000 μm.
The fuels for the high-temperature gas reactor may be produced in the following way with an apparatus for producing ammonium diuranate particles. Firstly, uranium oxide in the form of powder is dissolved in nitric acid, which produces a uranyl nitrate solution. Then, the uranyl nitrate solution is mixed with pure water, a thickening agent, and other chemicals, if necessary, and the mixture is stirred. A feedstock liquid to be dripped is obtained by this process. The feedstock liquid is stored in a feedstock liquid reservoir. The feedstock liquid thus prepared is cooled to a predetermined temperature, the viscosity thereof is adjusted, and then it is transferred to a dripping nozzle device. The dripping nozzle device has one nozzle with a small diameter. The transferred feedstock liquid falls in drops from the end of the nozzle into an aqueous solution of ammonia. The uranyl nitrate included in the drops, which have fallen into the aqueous solution of ammonia, changes into ammonium diuranate from the surfaces of the drops through the reaction. If the drops including uranyl nitrate reside in the solution for a time period enough to complete the reaction, uranyl nitrate in the central part of each drop is changed to ammonium diuranate.
The drops dripped from the nozzle pass through an atmosphere of ammonia gas in the process of falling toward the surface of the aqueous ammonia solution. This ammonia gas brings about gelation on the surface of each drop, which forms a film there. The drops with the film are protected from deformation to some extent, caused by the impact that occurs when the drops fall to and hit the surface of the aqueous ammonia solution. If uranyl nitrate included in the drops that have fallen into the solution reacts with ammonia sufficiently, ammonium diuranate particles, which may sometimes be abbreviated to “ADU particles”, are formed.
The ADU particles thus formed are washed, dried, and then calcined in the atmosphere, which changes the ADU particles into uranium trioxide particles. The obtained uranium trioxide particles are reduced and sintered, through which steps the uranium trioxide particles are changed into uranium dioxide particles with high density, in a condition like ceramics. The uranium dioxide particles are sieved, or classified, and fuel kernel particles with a diameter within a predetermined range are obtained.
One of the most important objectives to be achieved in the production of ADU particles is to produce ADU particles with almost the same diameter, with good sphericity or a shape that is very close to a sphere of perfect roundness, and without deficiencies inside each particle. In other words, the ADU particles are required to be of a uniform diameter, and free of deformations, made of ADU completely to the central part of each drop, and to have a flawless inside structure without cracks or other deficiencies. Another important objective is to produce ADU particles in a large quantity. In view of these objectives, the current ADU production apparatuses have a variety of problems to be solved as explained hereinafter.
In order to produce ADU particles, each of which has good sphericity, in a large quantity and a uniform quality, the dripping nozzle device should be capable of dripping the feedstock liquid so that the drops have the same volume.
However, technology has not seen such nozzles that are capable of dripping the feedstock liquid in drops with the same volume. Therefore have been desired the development of a dripping nozzle device capable of producing ADU particles with the same diameter and good sphericity, and that of an apparatus for producing ammonium diuranate particles, which may sometimes be called “apparatus for producing ADU particles” hereinafter.
The conventional apparatus for producing ADU particles comprises a dripping nozzle device with a single dripping nozzle, and the number of the produced ADU particles depends on how many times the nozzle vibrates. The number is 200 particles per second at most. In order to improve the productivity, the number of the nozzles has to be increased. When several dripping nozzles are employed, each dripping nozzle has to drip the same amount of the feedstock liquid. Dripping nozzle devices with several dripping nozzles have not been developed.
In order to produce ADU particles each of which has good sphericity and the same diameter in a large quantity, the dripping nozzles have to be capable of dripping the feedstock liquid so that the drops have the same volume.
However, technology has not seen such nozzle devices comprising nozzles that are capable of dripping the feedstock liquid in the same volume and allowing drops with the same volume to fall. Therefore have been desired the development of a dripping nozzle device to produce a large amount of uranium dioxide fuel kernels with the same diameter and good sphericity, and that of an apparatus for producing ADU particles.
With the conventional apparatus for producing ADU particles, the feedstock liquid remains in the feedstock liquid supplying pipe that transfers the feedstock liquid from the feedstock reservoir to the dripping nozzle when the dripping of the feedstock liquid from the nozzle is stopped. Then, when a fresh feedstock liquid is transferred from the reservoir to the nozzle through the feedstock liquid supplying pipe, the old feedstock liquid that has remained in the pipe is dripped into the aqueous ammonia solution.
The old feedstock liquid remaining in the pipe has different properties from the fresh one that has been stored in the reservoir under temperature control. This difference often causes deformation in ADU particles formed in the aqueous ammonia solution by the dripping, which leads to uranium dioxide particles, produced through the steps of aging the ADU particles, washing the aged, drying the washed, calcining the dried, reducing the calcined, and sintering the reduced, that do not satisfy a required sphericity, a required outer diameter of each particle, a required flawlessness of the inside structure, etc. This difference also results in a reduction in the yield of the produced uranium dioxide. We suppose that the afore-mentioned problem is caused by the state where the temperature of the remaining feedstock liquid is raised to room temperature and the viscosity thereof is decreased.
When a conventional apparatus for producing ADU particles was equipped with several nozzles to increase the production of the ADU particles, the flow rates of the feedstock liquid to be dripped from the respective nozzles had to be the same to make the ADU particles resulting from the drippings from the respective nozzles have the same predetermined volume. In order to meet this requirement, the conventional apparatus is provided with flow regulators and the dripping amount of the feedstock liquid from each nozzle is controlled. However, only with the flow regulators, it is difficult to make the flow rates of the feedstock liquid to be dripped from the nozzles identical, which results in ADU particles that do not have a uniform diameter.
The drops dripped from the nozzle or nozzles of the conventional apparatus for producing ADU particles fall toward the aqueous ammonia solution through an atmosphere of ammonia gas. Thus a film is formed on the surface of each drop due to gelation made during the falling and before reaching the surface of the aqueous ammonia solution. The film is not strong enough to prevent the drop from deformed by the impact given to the drop when it splashes down. Drops are sometimes even broken by the impact given when they hit the aqueous ammonia solution. A reaction between the deformed or broken particles of uranyl nitrate and ammonia in the aqueous ammonia solution does not lead to the production of ADU particles with good sphericity at a high yield. Also, when an apparatus for producing ADU particles with several dripping nozzles is employed and ammonia gas is sprayed over the drops dripped from the several nozzles and falling toward the solution, it is difficult to uniformly spray each drop with the gas and the drops often had wave-like patterns caused by the sprayed gas on the surfaces thereof.
The aqueous ammonia solution has to permeate into the center of each drop including uranyl nitrate dripped from the nozzle(s) of the conventional apparatus for producing ADU particles.
For the permeation of the aqueous ammonia solution is typically used an aqueous ammonia solution reservoir that contains the aqueous ammonia solution. In other words, an aqueous ammonia solution reservoir in which the aqueous ammonia solution is stored is placed right below the nozzle(s) that drip(s) the feedstock liquid including uranyl nitrate. The drops formed by dripping the feedstock liquid from the nozzle(s) of the nozzle device are allowed to fall into the aqueous ammonia solution stored in the aqueous ammonia solution reservoir.
Then, uranyl nitrate existing in the drops and ammonium ions react with each other in the solution to produce ammonium diuranate (ADU) in the drops.
The reaction between uranyl nitrate in a drop and ammonium ions starts from the surface of the drop, and then progresses to inner places of the drop as time passes. However, as the reaction between uranyl nitrate existing in the vicinity of the inner side of the surface of the drop and ammonia existing in the vicinity of the outer side of the surface of the drop proceeds, the concentration of ammonia existing in the vicinity of the outer side of the surface of the drop decreases. The decrease slows the reaction between uranyl nitrate and ammonium ions. Besides, in order for the ammonium ions to react with uranyl nitrate existing in the center of the drop, the ammonium ions existing outside the surface of the drop have to penetrate into the drop, and diffuse and move into the central part thereof. Therefore, it takes a long time to change uranyl nitrate in the center of the drop to ammonium diuranate, which is one problem. Another problem is that the reaction between uranyl nitrate and ammonium ions is often insufficient under the conditions where the drops stay still in the aqueous ammonium solution for a predetermined period of time. Due to these problems, with the conventional apparatus for producing ADU particles, it is difficult to produce ADU particles with a large diameter, which sometimes results in inferior fuel kernels with pores inside, obtained after the treatments in the subsequent steps.
Still another problem is that ADU particles with their central parts remaining unreacted have jelly-like central parts and that the ADU particles are very soft. Therefore when these ADU particles, the specific gravity of which is large because they include uranium, are piled up and accumulated, ADU particles located near and at the bottom of the aqueous ammonia solution reservoir become deformed, which results in the production of a large quantity of inferior particles with bad sphericity.
The object of the present invention is to solve the afore-mentioned problems.
One objective of the present invention is to provide a dripping nozzle device or dripping nozzle devices capable of supplying a feedstock liquid to a dripping nozzle or dripping nozzles at the same and constant flow rate, and allowing the feedstock liquid to fall in drops that include uranyl nitrate so that ADU particles with the same shape and size are produced in large quantities; and an apparatus for producing ADU particles that employs the dripping nozzle device or dripping nozzle devices as a part of it.
Another objective of the present invention is to provide a device for recovering a feedstock liquid, which is capable of solving the afore-mentioned conventional problems and producing ADU particles of uniform size and free of deformation at a high yield; and an apparatus for producing ADU particles that employs the device for recovering a feedstock liquid as a part of it.
Still another objective of the present invention is to provide a device for supplying a feedstock liquid to a dripping nozzle or nozzles that allow the feedstock liquid including uranyl nitrate to fall in drops, so that the afore-mentioned problems are solved and ADU particles of uniform size are produced; and an apparatus for producing ADU particles that employs the device for supplying a feedstock liquid as a part of it.
A further objective of the present invention is to provide a device for solidifying the surfaces of drops capable of appropriately solidifying the surfaces of drops including uranyl nitrate, so that the afore-mentioned problems are solved and ADU particles with good sphericity are produced; and an apparatus for producing ADU particles that employs the device for solidifying the surfaces of drops as a part of it.
A still further objective of the present invention is to provide a device for circulating an aqueous ammonia solution, capable of solving the afore-mentioned problems and producing ammonium diuranate particles with good sphericity and without deficiencies in the inside structure thereof; and an apparatus for producing ADU particles that employs the device for circulating an aqueous ammonia solution as a part thereof.